<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>计算器练习</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f1(1)">
<input type="button" value="减" onclick="f2(2)">
<input type="button" value="乘" onclick="f3(3)">
<input type="button" value="除" onclick="f4(4)">
<h1>结果:<span></span></h1>
<script>
    let i1 = document.querySelector("#i1");
    let i2 = document.querySelector("#i2");
    let s = document.querySelector("span");

    function f1(x) {
        //Not a Number:不是一个数  当NaN和任何数值进行任何运算时结果都为NaN
        if (isNaN(i1.value) || isNaN(i2.value)||i1.value.trim()==""||i2.value.trim()=="") {
            s.innerText = "输入错误!";
            return;
        }
        switch (x) {
            case 1:
                s.innerText = i1.value * 1 + i2.value * 1;
                break;
            case 2:
                s.innerText = i1.value - i2.value;
                break;
            case 3:
                s.innerText = i1.value * i2.value;
                break;
            case 4:
                s.innerText = i1.value / i2.value;


        }
    }
</script>
</body>
</html>